Lignite (brown coal) is a significant Australian energy resource (major deposits are located throughout Victoria, South Australia and Western Australia). The moisture content of lignite, commonly in the range 60-70% wt, w.b., is generally considered as the major economic barier to its commercial exploitation. Unfortunately, this high moisture lignite is the only solid fossil-fuel resource in Victoria. Taking advantage of the low cost of mining and the huge readily recoverable resources containing several hundred years of supply, Victoria has utilised this high-moisture low energy resource to generate the majority of its electrical power over the last eighty years. In addition, it is a widely accepted view that the coal reserves will remain important to the State economy in the many years to come.
The thermal conversion efficiency of lignite is however rather low, which is exacerbated by the high moisture content of the fuel, leading to higher levels of greenhouse gas emissions than from plants of similar capacity fuelled with gas or even high-rank black coal. Reducing moisture or pre-drying is therefore the first and most important step in developing cost-effective technologies for power generation from this abundant resource and particularly for increasing the efficiency of existing plant.
Most of the total water (up to 70%, as-received basis) in the as-mined lignite is contained in the capillary macrostructure and colloidal microstructure within the coal together with a small portion bound to the solid organic structure through electrostatic attraction and adsorption. Numerous techniques have been investigated for drying high-moisture coals, which can be broadly classified into two main categories—thermal drying and mechanical dewatering.
The thermal methods can be further divided into two groups—evaporative and non-evaporative. Evaporative processes involve applying heat to vaporise water from coal at atmospheric pressure. The heat transfer medium is usually hot flue gas, air or steam, applied either directly or indirectly. The energy requirements of these thermal drying processes involve inputs in the order of 3000-4000 kJ per kg of removed water, which are far greater than the enthalpy required for the evaporation of clean water. Non-evaporative thermal drying (also known as thermal dewatering) methods use steam or hot water to heat the coal up to the corresponding saturated steam pressure and the coal water is then removed as a liquid before cooling and/or depressurising. The energy requirements for non-evaporative thermal drying processes are much less (e.g. 1600-1800 kJ/kg of water) compared to the evaporative methods. This is possible as the enthalpy for evaporating the water can be dispensed through the use of increased pressure and the corresponding saturated steam temperatures to remove the moisture in the liquid form. The Fleissner and Hydro-Thermal Drying (HTD) processes are the most common non-evaporative thermal drying methods in use today.
Mechanical dewatering methods refer to the application of mechanical pressure to separate moisture from raw materials. The dewatered product can generally be created with a significantly lower energy input compared to known thermal methods. Mechanical methods have been widely used for fibrous materials (pulp and paper, citrus peel, sewage sludge, food product, animal waste, etc.) and to a lesser extent for coal slurries containing a high proportion of fines, such as minerals and coal dressings. Screw and disc presses have been investigated for the dewatering of brown coal in laboratory-scale test rigs. Banks P. J. & Burton D. R. ‘Press dewatering of brown coal’, Part 1, Drying Technology 7(2), 1989 discloses a study of the feasibility of upgrading Victorian brown coal via mechanical dewatering of as-mined coal and coal water slurries. Using a water permeable cell they found that the compressibility and permeability of solid coal decreased greatly during mechanical expression and that the coal displayed viscoelastic properties. Using pressures up to 140 bar, the product moisture content reduced to below 80% (by weight on a dry basis) and was shown to be strongly dependent on the applied pressure, irrespective of the initial moisture content. The coal became consolidated and the volume change recorded during the consolidation was largely irreversible. The moisture in the expressed coal was shown to decrease linearly with increasing pressure when plotted on a semi-logarithmic scale. Murase T et al. ‘Press dewatering of brown coal’, Part 2, Drying Technology 7(2), 1989 discloses a study of both batchwise and continuous dewatering processes using screw presses. The degree of dewatering was shown to be dependent on both the screw rotational speed and residence time of the material inside the barrel. The extent of dewatering was only significant when the screw speed was below a critical value (usually below 10 rpm) which was in turn determined by the equipment and the material properties. The product throughput was ultimately limited by this speed dependency.
Recent developments in the dewatering of lignite have utilised a method combining both thermal and mechanical treatments. This method, commonly known as the mechanical-thermal expression (MTE) process for dewatering high moisture brown coal and non-coal materials, requires that the coal be both heated and subjected to compression. The heating of the coal is typically achieved by injecting superheated steam into the coal. Pressure has typically been applied to the coal by use of a flat bed press, similar to those used in the manufacture of particle boards.
U.S. Pat. No. 4,702,745 discloses a process for dewatering coal by heating the coal in water at an elevated temperature and pressure in a heating chamber in order to reduce the moisture content of the coal. After pre-heating, the heated coal is subjected to compression by a compressing device while maintaining the temperature and the pressure of the water before lowering the pressure of the water in a decompressing chamber and then expelling a dewatered coal product. A problem with this dewatering process is that it can be difficult to feed the heated coal into the compressing device due to back-flow of material as the compressing device undergoes a compressing stroke and material is pushed back toward a heating vessel.
It is an aim of the present invention to provide a method and apparatus to facilitate feeding of a dewatering device and to ameliorate at least one or more disadvantages of the prior art.